EP1571826A2 - Schwellenmatrix zur Erzeugung von Rastern, Verfahren zur Erzeugung und zur Zuweisung solcher Matrix - Google Patents

Schwellenmatrix zur Erzeugung von Rastern, Verfahren zur Erzeugung und zur Zuweisung solcher Matrix Download PDF

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Publication number
EP1571826A2
EP1571826A2 EP05004587A EP05004587A EP1571826A2 EP 1571826 A2 EP1571826 A2 EP 1571826A2 EP 05004587 A EP05004587 A EP 05004587A EP 05004587 A EP05004587 A EP 05004587A EP 1571826 A2 EP1571826 A2 EP 1571826A2
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EP
European Patent Office
Prior art keywords
separation
threshold
dot
dots
threshold matrix
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EP05004587A
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English (en)
French (fr)
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EP1571826A3 (de
Inventor
Yoshiaki C/O Fuji Photo Film Co. Ltd. Inoue
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Fujifilm Corp
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Fujifilm Corp
Fuji Photo Film Co Ltd
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Publication of EP1571826A2 publication Critical patent/EP1571826A2/de
Publication of EP1571826A3 publication Critical patent/EP1571826A3/de
Withdrawn legal-status Critical Current

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/46—Colour picture communication systems
    • H04N1/52—Circuits or arrangements for halftone screening
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/40—Picture signal circuits
    • H04N1/405—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels
    • H04N1/4051—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels producing a dispersed dots halftone pattern, the dots having substantially the same size

Definitions

  • the present invention relates to a threshold matrix, a method of generating such a threshold matrix and a method of assigning such a threshold matrix that are suitable for use in a printing-related apparatus (output system) such as a filmsetter, a platesetter, a CTP (Computer To Plate) apparatus, a CTC (Computer To Cylinder) apparatus, a DDCP (Direct Digital Color Proof) system, etc., an ink jet printer, or an electrophotographic printer.
  • a printing-related apparatus such as a filmsetter, a platesetter, a CTP (Computer To Plate) apparatus, a CTC (Computer To Cylinder) apparatus, a DDCP (Direct Digital Color Proof) system, etc.
  • a printing-related apparatus such as a filmsetter, a platesetter, a CTP (Computer To Plate) apparatus, a CTC (Computer To Cylinder) apparatus, a DDCP (Direct Digital Color Proof) system, etc.
  • an ink jet printer
  • the dot pattern in which no screen ruling or screen angle is defined refers not to a general AM screen (including a line screen etc.) having halftone dots with the screen ruling and screen angle being uniquely determined, but to a pattern that is generally categorized as an FM screen or a stochastic screen.
  • AM Amplitude Modulation
  • FM Frequency Modulation
  • a process of generating a threshold matrix for FM screens is disclosed in Japanese Laid-Open Patent Publication No. 8-265566.
  • an array of elements of a threshold matrix i.e., an array of thresholds is generated in an ascending order or a descending order by determining threshold positions such that the position of an already determined threshold is spaced the greatest distance from the position of a threshold to be newly determined.
  • the dot pattern of a binary image that is generated using the threshold matrix thus produced has dots which are not localized. Even when a dot pattern is generated using a plurality of such threshold matrixes that are juxtaposed, the dot pattern does not suffer a periodic pattern produced by the repetition of threshold matrixes.
  • Japanese Patent No. 3400316 discloses a method of correcting halftone image data by extracting a pixel having a weakest low-frequency component of a certain dot pattern, from white pixels (unblackened pixels), and a pixel having a strongest low-frequency component of the dot pattern, from blackened pixels, and switching around the extracted white and blackened pixels.
  • the dot pattern is intended to be smoothed or leveled.
  • Japanese Laid-Open Patent Publication No. 2001-292317 reveals a process of determining threshold positions in a threshold matrix such that a next blackened pixel is assigned to a position having a weakest low-frequency component of the threshold matrix.
  • Japanese Laid-Open Patent Publication No. 2002-368995 shows a process of determining threshold positions in a threshold matrix such that when an array of thresholds in the threshold matrix has been determined up to a certain gradation and a threshold position for a next gradation is to be determined, blackened pixels are assigned to positions for not strengthening a low-frequency component.
  • Japanese Laid-Open Patent Publication No. 2002-369005 discloses a process of generating a threshold matrix according to the process shown in Japanese Patent No. 3400316, Japanese Laid-Open Patent Publication No. 2001-292317, or Japanese Laid-Open Patent Publication No. 2002-368995 based on an ideal dot pattern at a certain gradation which is given.
  • FM screens When an FM screen is used for offset printing, it causes shortcomings in that the quality of printed images suffers some grainness. FM screens also cause disadvantages in that a dot gain tends to become large and images are reproduced unstably when images are printed, or when films are output in an intermediate printing process, or when a printing plate is output by a CTP apparatus.
  • a dot size is determined to be the size of a dot made up of one pixel or a dot made up of four pixels according to a 1 (1 ⁇ 1)-pixel FM screen or a 4 (2 ⁇ 2)-pixel FM screen
  • an array of thresholds of a threshold matrix is determined by an algorithm for generating FM screens, thus determining an output quality, and only the dot size serves as a parameter for determining the quality of FM screens.
  • a dot size is determined to be a 3 ⁇ 3-pixel FM screen dot size with respect to an output system which is incapable of stably reproducing 2 ⁇ 2-pixel FM screen dots for highlight areas, then the resolution (referred to as pattern frequency or pattern resolution) for intermediate tones is lowered, resulting in a reduction in the quality of images.
  • FIG. 22 of the accompanying drawings shows a conventional dot pattern 1 in a highlight area where the dot percentage of a 2 ⁇ 2-pixel FM screen is 5%, a conventional dot pattern 2 in an intermediate tone area where the dot percentage of the 2 ⁇ 2-pixel FM screen is 50%, a conventional dot pattern 3 in a highlight area where the dot percentage of a 3 ⁇ 3-pixel FM screen is 5%, and a conventional dot pattern 4 in an intermediate tone area where the dot percentage of the 3 ⁇ 3-pixel FM screen is 50%.
  • FIG. 23 of the accompanying drawings shows a power spectrum generated when the dot pattern 2 of the 2 ⁇ 2-pixel FM screened shown in FIG. 22 is FFTed (Fast-Fourier-Transformed), and FIG. 24 of the accompanying drawings shows a power spectrum generated when the dot pattern 4 of the 3 ⁇ 3-pixel FM screen shown in FIG. 22 is FFTed.
  • the dot pattern 2 of the 2 ⁇ 2-pixel FM screen suffers less grainness than the dot pattern 4 of the 3 ⁇ 3-pixel FM screen, but has the dot percentage less reproducible in the printed image.
  • the dot pattern 4 of the 3 ⁇ 3-pixel FM screen has a pattern frequency 6 of about 13 c/mm which is lower than the pattern frequency 5 of about 20 c/mm of the dot pattern 2 of the 2 ⁇ 2-pixel FM screen.
  • the pattern frequencies 5, 6 which are of peak values are also called a peak spatial frequency fpeak.
  • the dot size of the 1 ⁇ 1 pixel FM screen is 10 ⁇ m ⁇ 10 ⁇ m (10.6 ⁇ m ⁇ 10.6 ⁇ m)
  • the dot size of the 2 ⁇ 2 pixel FM screen is 20 ⁇ m ⁇ 20 ⁇ m (21.2 ⁇ m ⁇ 21.2 ⁇ m).
  • the output resolution R is different from the pattern frequencies 5, 6 of the dot patterns 2, 4 shown in FIGS. 23, 24.
  • dot patterns representative of a v-valued image in which no screen ruling or screen angle is defined is preferable in superimposing, rather than an AM screen having halftone dots with the screen ruling and screen angle being uniquely determined. It is known that shortcomings due to a periodic pattern in superimposing the plates do not often occur when such dot patterns representative of a v-valued image are used.
  • threshold matrixes each having a different threshold array is generated for each plate.
  • it is quite difficult to generate threshold matrixes each having a different threshold array and it requires a heavy workload.
  • it is difficult to handle five or more thresholds for color plates in a RIP system etc. generally using four threshold matrixes for four C, M, Y and K colors.
  • the generated threshold matrix is optimum for use in an output system, and is capable of reproducing high-quality images of excellent printability.
  • Another object of the present invention is to provide a method of assigning a threshold matrix which is capable of generating threshold matrixes for five or more color separations for reproducing a color image with a light workload, and which is capable of generating threshold matrixes that do not cause shortcomings in superimposing images.
  • a method of generating a threshold matrix for converting a continuous-tone image into a dot pattern representing a binary image comprising the steps of:
  • the number of pixels of a dot of a minimum size and a pattern frequency at a dot percentage of an intermediate tone are determined. Based on the number of pixels of a dot of a minimum size and a pattern frequency, candidate positions for the dots in the dot pattern for highlight areas and shadow areas so that the pattern frequency is provided at the dot percentage. Then, the number of new dots of the minimum size is determined at a next dot percentage with respect to a present dot percentage for which the dot pattern has already been determined. Subsequently, the number of pixels of each of the dots is adjusted, i.e., increased or decreased so that the dot pattern has the next dot percentage. Finally, each of the thresholds is set so that the dot pattern whose number of pixels is adjusted is generated. Thus, the generated threshold matrix is optimum for use in an output system.
  • the step of determining the candidate positions for the dots in the dot pattern may comprise the steps of: determining the dot pattern of the intermediate tone so that the dot pattern has the pattern frequency; and using the determined dot pattern of the intermediate tone as the candidate positions for the dots of the minimum size in the dot pattern at the next dot percentage.
  • the intermediate tone has dot percentages in a range from 10% to 90%.
  • the step of determining the number of the new dots of the minimum size at the next dot percentage comprises the steps of: in a dot percentage range from 0% to 50%, determining the number of the new dots such that the number of the dots in the dot pattern is gradually reduced from the number of the dots corresponding to an ideal FM screen in a dot percentage range from 0% to a certain percentage; and determining the number of the new dots such that the number of the new dots is zero in a dot percentage range from the certain percentage to 50%.
  • the step of determining the number of the new dots of the minimum size at the next dot percentage comprises the steps of: in a dot percentage range from 0% to 50%, determining the number of the new dots such that the number of the dots in the dot pattern is gradually reduced from the number of the dots corresponding to an ideal FM screen in a dot percentage range from 0% to a first percentage; determining the number of the new dots such that the number of the new dots is zero in a dot percentage range from the first percentage to a second percentage; and determining the number of the new dots such that the number of the dots in the dot pattern is gradually increased in a dot percentage range from the second percentage to 50%. Accordingly, a predetermined pattern frequency can be obtained in intermediate tones, and the dot pattern whose dot gain is small can be obtained.
  • low-frequency components may be extracted from the dot pattern that is generated by a threshold matrix having thresholds whose placement positions have already been determined.
  • the placement positions of thresholds may be adjusted so that a pixel is added to the existing dots in positions where the extracted low-frequency components are weakest.
  • the placement positions of thresholds may be adjusted so that a pixel is deleted from the existing dots in positions where the extracted low-frequency components are strongest.
  • a density image (density dot pattern) corresponding to a dot pattern reproduced on a recording medium such as a film, a printing plate, or printing paper may be predicted by calculating based on the shape of the laser beam and the characteristics of photosensitive material. Then, low-frequency components may be extracted from the predicted density image (density dot pattern).
  • the increase in the dot periphery length is suppressed compared with a conventional FM screen.
  • dot gain is reduced, and the threshold matrix is optimum for use in an output system, and reproducing high-quality images of excellent printability is achieved.
  • the threshold matrix for converting a continuous-tone image into a dot pattern representing a binary image
  • the dot pattern is generated such that dots of a predetermined minimum size which are made up of n pixels (n is at least 1) are placed out of contact with each other at dot percentages from 0% to a certain percentage where the number of dots becomes nearly N ⁇ M/(R/r) 2
  • the dot pattern is generated such that pixels are attached to a periphery of existing dots of the minimum size to adjust the dot areas.
  • the dot pattern is generated by the threshold matrix such that new dots of the minimum size will be increased until the number of the dots becomes a certain value that corresponds to the pattern frequency r. Then, the pattern frequency is substantially held in the intermediate tones.
  • the threshold matrix that is capable of generating a dot pattern having a pattern frequency close to a target pattern frequency can be obtained.
  • the number of pixels of a dot of a minimum size, a pattern frequency of an intermediate tone, the number of new dots of the minimum size at each dot percentage are determined. Based on these, placement positions for thresholds in the threshold matrix are successively determined, and the threshold matrix that is optimum for use in an output system can be generated.
  • the image can be stably reproduced for highlight areas, suitability for output and printability can be improved, and high-quality images can be reproduced at all dot percentages.
  • the pattern frequency is set to a sufficiently small value without large values, the threshold matrix that generates a dot pattern having suitability for output without sudden rise in dot gains can be generated.
  • the generated threshold matrix is capable of generating an image where dots are reliably and solidly assigned to a highlight area, and where grainness is reduced and a dot gain is small in an intermediate tone area.
  • the threshold matrix may be stored in a storage unit as data.
  • a raster image processor may have the storage unit for storing the threshold matrix as data.
  • a method of assigning threshold matrixes to color separations each of the threshold matrixes converting continuous-tone image data into dot pattern image data, the continuous-tone image data comprising at least data for J colors (J ⁇ 5) including C, M, Y and K, the dot pattern image data comprising data for J color separations in which screen ruling or screen angle is not defined, the method comprising the steps of: assigning a first threshold matrix with a first threshold array to a first separation for a first color, the first color being K; assigning a second threshold matrix with a second threshold array to a second separation for a second color other than K; and assigning a third threshold matrix with a third threshold array to a third separation for a third color, the third color being adjacent to the second color in a hue circle, the number of the threshold matrixes assigned to the J color separations being as small as possible.
  • the three different threshold matrixes are assigned to a separation for K-color, a separation for another color other than K, and a separation for the color adjacent to the other color in the hue circle, respectively.
  • a color component of a substantially fan-shaped area defined by boundaries of K-color, the color other than K, and the color adjacent to the other color can be reproduced by mixing these three colors.
  • threshold matrixes having different threshold arrays are used for these three colors to be mixed, excessive overlapping of dots are avoided and shortcomings due to superimposition of images can be prevented.
  • workload of generating the threshold matrixes can be reduced.
  • the J color separations are made up of a C-separation, an M-separation, a Y-separation, a K-separation, an R-separation, a G-separation and a B-separation.
  • C is adjacent to G
  • G is adjacent to Y
  • Y is adjacent to R
  • R is adjacent to M
  • M is adjacent to B
  • B is adjacent to C.
  • the second threshold matrix is assigned to one of the C-separation, Y-separation and the M-separation.
  • C is not adjacent to Y
  • Y is not adjacent to M
  • M is not adjacent to C.
  • the third threshold matrix is assigned to the G-separation, R-separation and the B-separation, and in the hue circle, G is not adjacent to R, R is not adjacent to B, and B is not adjacent to G. Accordingly, for the separations for the colors of C, M, Y, K, R, G and B, only three threshold matrixes each having a different threshold array are sufficient. The workload of generating the threshold matrixes can be reduced. In this way, it is possible to handle threshold matrixes for separations for C, M, Y, K, R, G and B colors in a RIP system etc. generally using four threshold matrixes for four separations for C, M, Y and K colors.
  • the J color separations are made up of a C-separation, an M-separation, a Y-separation, a K-separation, an O-separation and a G-separation.
  • C is adjacent to G
  • G is adjacent to Y
  • Y is adjacent to O
  • O is adjacent to M
  • M is adjacent to C.
  • the second threshold matrix is assigned to the C-separation
  • the third threshold matrix is assigned to the G-separation
  • the second threshold matrix is assigned to the Y-separation
  • the third threshold matrix is assigned to the O-separation
  • a fourth threshold matrix with a fourth threshold array is assigned to the M-separation.
  • the threshold matrix for the M-separation is the same as the threshold matrix for the C-separation, which is undesirable. Accordingly, for the separations for the colors of C, M, Y, K, O and G, only four threshold matrixes each having a different threshold array are sufficient. The workload of generating the threshold matrixes can be reduced. In this way, it is possible to handle threshold matrixes for separations for C, M, Y, K, O and G colors in a RIP system etc. generally using four threshold matrixes for four separations for C, M, Y and K colors.
  • a method of assigning threshold matrixes to color separations each of the threshold matrixes converting continuous-tone image data into dot pattern image data, the continuous-tone image data comprising at least data for J colors (J ⁇ 5) including C, M, Y and K, the dot pattern image data comprising data for J color separations in which screen ruling or screen angle is not defined, the method comprising the step of: assigning a first threshold matrix to a separation for a color other than C, M, Y or K, the first threshold matrix being different from a threshold matrix for C, M or Y that is adjacent to the color in a hue circle.
  • the number of the threshold matrixes for reproducing a color image with five or more color separations may generally be four, i.e., the threshold matrixes for the C-separation, the M-separation, the Y-separation and the K-separation.
  • the threshold matrixes that do not cause shortcomings in superimposing images can be generated and assigned with a light workload.
  • a method of assigning threshold matrixes to color separations each of the threshold matrixes converting continuous-tone image data into dot pattern image data, the continuous-tone image data comprising at least data for J colors (J ⁇ 5) including C, M, Y and K, the dot pattern image data comprising data for J color separations in which screen ruling or screen angle is not defined, the method comprising the steps of: assigning a first threshold matrix with a first threshold array to a K-separation; and assigning a second threshold matrix with a second threshold array to separations for colors other than K, the colors other than K being complementary to each other.
  • threshold matrixes having the same threshold array are used for separations for the hues that are complementary to each other, unstable color reproduction or unevenness or irregularity of hue or shade in the image due to less superimposition of dots by screen displacement does not occur. This is because the colors that are complementary to each other are seldom mixed. If the colors that are complementary to each other are mixed, the mixed color is merely gray. Further, workload of generating the threshold matrixes can be reduced.
  • the J color separations are made up of a C-separation, an M-separation, a Y-separation, a K-separation, an R-separation, a G-separation and a B-separation.
  • the second threshold matrix is assigned to the C-separation
  • a third threshold matrix is assigned to the M-separation
  • a fourth threshold matrix is assigned to the Y-separation.
  • C is adjacent to G
  • G is adjacent to Y
  • Y is adjacent to R
  • R is adjacent to M
  • M is adjacent to B
  • B is adjacent to C.
  • the third threshold matrix for the M-separation is assigned to the G-separation
  • the second threshold matrix for the C-separation is assigned to the R-separation
  • the fourth threshold matrix for the Y-separation is assigned to the B-separation
  • G is complementary to M
  • R is complementary to C
  • B is complementary to Y. Accordingly, for the separations for the colors of C, M, Y, K, R, G and B, only four threshold matrixes each having a different threshold array, e.g., for four separations generally for C, M, Y and K colors are sufficient. Thus, workload of generating the threshold matrixes can be reduced.
  • the J separations are made up of a C-separation, an M-separation, a Y-separation, a K-separation, an O-separation and a G-separation.
  • the second threshold matrix is assigned to the C-separation
  • a third threshold matrix is assigned to the M-separation
  • a fourth threshold matrix is assigned to the Y-separation.
  • C is adjacent to G
  • G is adjacent to Y
  • Y is adjacent to O
  • O is adjacent to M
  • M is adjacent to C.
  • the third threshold matrix for the M-separation is assigned to the G-separation
  • the second threshold matrix for the C-separation is assigned to the O-separation
  • G is complementary to M
  • O is complementary to C. Accordingly, for the separations for the colors of C, M, Y, K, O and G, only four threshold matrixes each having a different threshold array, e.g., for four separations generally for C, M, Y and K colors are sufficient. Thus, workload of generating the threshold matrixes can be reduced.
  • one of the threshold matrixes is generated by changing a reading method of thresholds placed in the threshold array in other of the threshold matrixes.
  • a threshold array in the threshold matrix can be changed, and time for generating a threshold matrix can be significantly reduced, compared with a threshold matrix generated from nothing.
  • threshold matrixes are generated for five or more color separations for reproducing a color image with dot patterns in which screen ruling or screen angle is not defined.
  • the threshold matrixes each having a different threshold array are used for the separations for hues that are adjacent to each other in a hue circle, and the number of the threshold matrixes is made as small as possible.
  • the threshold matrixes can be generated and assigned with a light workload.
  • threshold matrixes are generated for five or more color separations for reproducing a color image with dot patterns in which screen ruling or screen angle is not defined.
  • threshold matrixes having the same threshold array are assigned to separations for hues that are complementary to each other in a hue circle.
  • the threshold matrixes can be generated with a light workload, and assigned so as not to cause shortcomings in superimposing images.
  • FIG. 1 shows a basic arrangement of a threshold matrix generating system 10 according to an embodiment of the present invention.
  • the threshold matrixes TM generated by the threshold matrix generating apparatus 20 are a threshold matrix for an FM screen, for example, to convert the continuous-tone image data I with the tone value u into the dot pattern in which no predetermined screen ruling or screen angle is defined.
  • the threshold matrix storage unit 14 comprises a recording medium such as a hard disk or the like.
  • the image data generator 12, the comparator 16, the dot pattern generator 18, and the threshold matrix generating apparatus 20 may comprise function realizing means that are achieved when a program stored in a personal computer (including a CPU, a memory, an input unit 20a such as a keyboard, a mouse, etc., and an output unit such as a display unit 20b, a printer 20c, etc.) is executed by the computer.
  • the function realizing means of the threshold matrix generating apparatus 20 may comprise a piece of hardware. An arrangement and operation of the function realizing means of the threshold matrix generating apparatus 20 will be described later on.
  • the output system 22 basically comprises a CTP apparatus having an exposure unit 26 and a drum 27 with printing plate materials EM wound thereon.
  • the exposure unit 26 applies a plurality of laser beams (recording beams), which are turned on and off for each pixel depending on the dot pattern data Ha, to the printing plate materials EM on the drum 27 that is being rotated in a main scanning direction MS by a main scanning motor (not shown) at a high speed, while the exposure unit 26 is being moved in an auxiliary scanning direction AS along the axis of the drum 27 by an auxiliary scanning motor (not shown).
  • a dot pattern representing a two-dimensional image as a latent image is formed on each of the printing plate materials EM.
  • the laser beams applied to the printing plate materials EM may be in several hundred channels.
  • the printing plate materials EM (usually, four printing plate materials with different screen angles for C, M, Y, K printing plates) on which the dot patterns are formed as latent images are developed by an automatic developing machine 28, producing printing plates PP with visible dot patterns formed thereon.
  • the produced printing plates PP are mounted on a printing press (not shown), and inks are applied to the mounted printing plates PP.
  • the printing plate materials EM contain a photosensitive material which should preferably be a positive image recording material including an alkaline dissolvable binder, a substance for generating heat upon absorption of an infrared radiation or a near-infrared radiation, and a thermally decomposable substance for substantially lowering the dissolvability of the binder when not thermally decomposed, as disclosed in Japanese Patent No. 3461377.
  • the printing plates PP should preferably be made of an image recording material including a photosensitive material which comprises a support base such as an aluminum sheet, a polyester film, or the like, and a layer including the above substances and mounted on the support base.
  • the alkaline dissolvable binder contains a phenolic resin, an acrylic resin, or a polyurethane resin.
  • the substance for generating heat upon absorption of an infrared radiation or a near-infrared radiation comprises a dye, a pigment, or carbon black.
  • the thermally decomposable substance for substantially lowering the dissolvability of the binder when not thermally decomposed comprises onium salt, diazonium salt, or a substance containing a quinone diazide compound.
  • the inks applied to the printing plates PP are transferred to a printing sheet as a recording medium such as a photographic sheet or the like, a desired printed material comprising an image formed on the printing sheet is obtained.
  • the output system 22 is not limited to the scanning exposure apparatus employing laser beams, but may be an apparatus for forming an image on a film, a printing plate, or a printed material according to a planar exposure process or an ink jet process, or a CTC printing machine.
  • the threshold array of the threshold matrixes TM stored in the threshold matrix storage unit 14 can be recorded and carried around in a portable recording medium which is a packaged medium such as a DVD, a CD-ROM, a CD-R, a semiconductor memory, or the like.
  • a process of generating a threshold matrix using the threshold matrix generating system shown in FIG. 1 will be described below with reference to a flowchart of FIG. 2.
  • the process shown in FIG. 2 is based on a program which is mainly executed by the threshold matrix generating apparatus 20.
  • the first parameter represents the size of a threshold matrix TM to be stored in the threshold matrix storage unit 14, i.e., the size N ⁇ N of a threshold matrix TM which contains N ⁇ N thresholds corresponding to N ⁇ N pixels.
  • the threshold matrix TM contains thresholds th ranging from 0 to thmax at respective positions (elements) determined by addresses (x, y).
  • the maximum threshold thmax has a value that is set to "255" for a system having 8-bit gradations and "65535" for a system having 16-bit gradations.
  • the size N ⁇ N of a square threshold matrix will be described below.
  • the present invention is also applicable to the size N ⁇ M of an elongate rectangular threshold matrix.
  • a plurality of threshold matrixes TM having the same threshold array and matrix size N ⁇ N and laid out as tiles (referred to as a superthreshold matrix STM) are used depending on the size of an image to be processed.
  • the thresholds th of the threshold matrix TM is determined in view of the threshold array of the entire superthreshold matrix STM.
  • the size of a pixel that can be output from the output system 22 is represented by 10 ⁇ m ⁇ 10 ⁇ m, which corresponds to a 1 ⁇ 1-pixel dot or 1 pixel.
  • the size 10 ⁇ m ⁇ 10 ⁇ m is a minimum unit that can be controlled by the exposure unit 26 for recording image data on the printing plate materials EM.
  • the second parameter represents the number of pixels that make up a dot of a minimum size which can stably be output from the output system 22, or stated otherwise, can stably be formed on the printing plates PP which are output from the output system 22.
  • the dot of a minimum size may be set to a 1-pixel dot (the number of pixels that make up a dot of a minimum size is one), a 2-pixel dot, a 3-pixel dot, a 2 ⁇ 2-pixel (the number of pixels that make up a dot of a minimum size is four) dot, a 2 ⁇ 3-pixel (6-pixel) dot, a 3 ⁇ 3-pixel (9-pixel) dot, etc.
  • the third parameter represents the pattern frequency at a predetermined dot percentage (also referred to as density percentage) in intermediate tones having a dot percentage in the range from 10% to 50%, i.e., the pattern frequency r of an intermediate tone dot pattern.
  • the pattern frequency r of an intermediate tone dot pattern represents the peak spatial frequency fpeak c/mm of a dot pattern in an intermediate tone.
  • the peak spatial frequency fpeak is concerned with the reproduction of image details, and also affects image quality in terms of grainness.
  • step S2 a dot candidate position in a highlight area HL and a dot candidate position in a shadow area SD are determined to provide the pattern frequency r in an intermediate tone.
  • a white noise generator 30 generates a white noise pattern WH at a dot percentage of 50% having the same size N ⁇ N as the size N ⁇ N of the threshold matrix TM.
  • the white noise pattern WH is an image where 1-pixel dots are randomly positioned in a spatial domain.
  • the white noise pattern WH can be generated so as to have desired values in an intermediate tone having a dot percentage in the range from 10% to 90%.
  • the white noise pattern WH is FFTed by an FFT (Fast Fourier Transform) unit 32, and then subjected to a bandpass filtering process at the pattern frequency r ( ⁇ ⁇ ) by a pattern frequency bandpass filter (pattern frequency BPF) 34, producing ring-shaped frequency-domain data AFFT2 having a radius equal to the pattern frequency r, as shown in FIG. 3B.
  • FFT Fast Fourier Transform
  • pattern frequency BPF pattern frequency bandpass filter
  • the frequency-domain data AFFT2 is IFFTed by an IFFT (Inverse Fast Fourier Transform) unit 36, producing space-domain data A2 of a continuous-tone image, as shown in FIG. 3C.
  • IFFT Inverse Fast Fourier Transform
  • each of the pixels of the spatial-domain data A2 is compared with a central gradation value (e.g., 127 if the maximum gradation is 255) by a comparator 38, generating binary data A2_bin, as shown in FIG. 3D.
  • a central gradation value e.g., 127 if the maximum gradation is 255
  • blackened portions serve as dot candidate positions in highlight areas HL and white portions (areas) serve as dot candidate positions in shadow areas SD.
  • the binary data A2_bin represent candidate positions for placing dots in highlight areas HL or the shadow areas SD.
  • the pattern of the binary data A2_bin may not necessarily be produced when the dot percentage is 50%.
  • the pattern may be changed for achieving the optimum dot pattern.
  • a 50% dot pattern can be established when a characteristic dot pattern is to be used at the dot percentage of 50% or when the dot pattern corresponding to the binary data A2_bin can be corrected into an optimum 50% dot pattern.
  • step S3 the number Dn of dots of a minimum size (also referred to as the number of dots of a new minimum size dots or the number of new dots of a minimum size) to be newly set at a present dot percentage is determined with respect to the dot percentage for which a dot pattern has been determined.
  • step S3 when candidate positions for dots are successively determined as the dot percentage is incremented, the number Dn(P) of dots of a minimum size to be newly established at a present dot percentage P is determined with respect to the preceding dot percentage P - 1 for which a dot pattern has already been determined.
  • the vertical axis of the graph shown in FIG. 4 represents a calculated accumulated value Ds of the number Dn of dots of a minimum size to be newly established (the number of new dots). Actually, as the dot percentage P becomes greater than 25%, since adjacent dots of a minimum size become closer to each other, the actual number of dots in a dot pattern is smaller than the accumulated value of the number Dn of new dots shown in FIG. 4.
  • the threshold matrix produces a conventional FM screen, which causes disadvantages in that a dot gain tends to become large and images are reproduced unstably when images are printed or films are output in an intermediate printing process.
  • all dots comprise dots of a minimum size in those highlight areas.
  • the size of dots is increased from the minimum size, e.g., dots composed of 5 pixels (2 ⁇ 2 + 1) or more are used.
  • the number Dn of new dots to be established at each dot percentage is gradually reduced, as indicated by a broken-line curve nc which represents the accumulated value of the number of new dots.
  • the number Dn of new dots to be established at each dot percentage is set to zero.
  • the number Dn is gradually increased, as indicated by the dot-and-dash-line curve nb which represents the accumulated value of the number of new dots.
  • each side of the N ⁇ N-pixel area has to contain 20 blackened dots (one dot comprises 2 ⁇ 2 pixels with r c/mm) of a minimum size, each composed of 4 pixels per 100 pixels/mm (R pixels/mm).
  • the total number of pixels of a dot pattern generated by the threshold matrix TM at each dot percentage is the same as with the conventional FM screens, i.e., the dot percentage is the same, but the number of dots is smaller than with the conventional FM screens. Therefore, a periphery length representing the sum of the lengths of the peripheries of all the dots of the dot pattern is smaller than with the conventional FM screens.
  • the dot pattern 100 contains sixteen 1 ⁇ 1-pixel dots 102.
  • the dot pattern 104 contains four 2 ⁇ 2-pixel dots 106. Though the total area of the dots 102 of the dot pattern 100 and the total area of the dots 106 of the dot pattern 104 are the same as each other, the dot patterns 100, 104 have different periphery lengths.
  • the dot pattern 100 and the dot pattern 104 have the same dot percentage, the sum of the lengths of white/black boundaries per unit area of the dot pattern, i.e., the dot periphery length of the dot pattern 100, is twice as long as the periphery length of the dot pattern 104.
  • the number Dn of new dots of a minimum size is established such that it increases again a substantially constant number after the dot percentage exceeds 25% and until it reaches 50%. According to the curve nb, dots are prevented from contacting each other in the vicinity of the dot percentage of 50%, thus avoiding the occurrence of a tone jump.
  • the accumulated value Ds of the number Dn of new dots may be established according to a curve which is in symmetric relation to the curves nc, nb with respect to the vertical line at the dot percentage of 50%.
  • the curve is analyzed from 100% toward 50%, and the number of new dots of white pixels (2 ⁇ 2 white pixels) is considered rather than the number Dn of new dots of blackened pixels.
  • a process of determining thresholds th alternately successively in ascending and descending orders in the highlight area HL and the shadow area SD in step S4 will be described below with reference to a flowchart shown in FIG. 6.
  • the process of successively determining thresholds th in the highlight area HL will mainly be described below.
  • the same process of successively determining thresholds th is carried out.
  • positions (array) for placing all thresholds th up to the dot percentage of 50% are determined in the order of threshold 0 ⁇ threshold thmax ⁇ threshold 1 ⁇ threshold thmax-1 ⁇ ⁇ ⁇ threshold (thmax-1)/2.
  • dot center positions are established in step S12.
  • the dot center positions are determined such that the dots established (assigned) by the thresholds th_hl whose placement positions are to be determined in the present threshold matrix TM are established in positions most spaced from the presently existing dots determined by the thresholds th_hl-1 for the preceding gradation where the placement positions of the thresholds th in the threshold matrix TM have already been determined.
  • FIG. 7 shows a superthreshold matrix STM made up of nine threshold matrixes TM1 through TM9 each having 25 thresholds.
  • positions for placing thresholds are determined in an ascending order from the highlight areas HL of the threshold matrixes TM or in a descending order from the shadow areas SD thereof.
  • central positions of newly placed thresholds th_hl are determined such that the already determined positions for placing thresholds th ("1" in FIG. 7) and the positions for newly placing thresholds th_hl ("2" in FIG. 7) are most spaced from each other in the threshold matrixes TM including a central threshold matrix (a 5 ⁇ 5 threshold matrix in FIG. 7) TM5 and other threshold matrixes TM1 through TM4, TM6 through TM9 of the same threshold layout which are disposed around the central threshold matrix TM5 as nine nearby threshold matrixes in FIG. 7.
  • the central threshold "2" in a thick-line frame which is disposed within the threshold matrix TM5 is placed in either a position which contains a point contacted by four circles around respective four thresholds "1" in thick-line frames positioned around the central threshold "2" or a position which is closet to the above position and represents a blackened portion of the binary data A2_bin (see FIG. 3D).
  • positions 112 marked with ⁇ for example, in a dot pattern 110 which is made up of dots 108 based on the thresholds th determined up to present are determined as central positions for placing dots.
  • step S13 candidates (threshold candidates) th'_hl for positions for placing thresholds are established.
  • steps S14 through S16 it is determined whether the total number of pixels of a dot pattern generated by the threshold matrixes TM where the layout of the thresholds th is determined up to present, corresponds to the present dot percentage or not, thereby correcting the total number of pixels.
  • the dot pattern is generated as follows:
  • the image data generator 12 generates continuous-tone image data (image data I for generating a screen tint) of a gray pattern (whose pixel values are the same) corresponding to the dot percentage.
  • the comparator 16 compares the generated continuous-tone image data with the threshold matrixes TM stored in the threshold matrix storage unit 14 and including thresholds up to the threshold th-1 which have been determined up to present.
  • Binary data H produced from the comparator 16 are supplied to the dot pattern generator 18, which produces dot pattern data Ha.
  • a dot pattern based on the dot pattern data Ha is displayed on the display unit 20b or the like.
  • new threshold candidates th' are established as dots for adding those pixels from the dots that are not based on the existing thresholds 0 through th-1 or the dots that are not based on the newly established threshold candidates th'_hl whose placement positions have not yet been determined in step S15.
  • a few dots may possibly be smaller than dots of a minimum size.
  • the dots of a minimum size are 2 ⁇ 2-pixel dots
  • the total number of pixels of the dot pattern which is made up of the dots of a minimum size is a multiple of 4. If the total number of dots is adjusted in order to equalize dot percentages, 3-pixel dots, 2-pixel dots, or 1-pixel dots, which are produced by deleting one, two, or three pixels from each of 2 ⁇ 2-pixel dots, may be necessary.
  • a dot pattern (binary image data) in the spatial domain which is made up of the dots based on the thresholds 0 through th-1 whose placement positions have already been determined and the dots based on the newly established threshold candidates th'_hl is FFTed into a dot pattern in the frequency domain by the FFT unit 32, after which high frequencies in the dot pattern are cut off by an LPF (Low-Pass Filter) 40.
  • the dot pattern is IFFTed back into a dot pattern in the spatial domain by the IFFT unit 36, after which low-frequency components are extracted from the dot pattern.
  • Positions where the extracted low-frequency components are weakest are set to threshold candidates th' to be added. However, if a dot pattern having a dot percentage of 50% is established in step S2, then positions where the low-frequency components are weakest within blackened pixels of the dot pattern having the dot percentage of 50% may be set to threshold candidates th' to be added.
  • the low-frequency components are extracted by a human visual characteristic filter 42, used as the LPF 40, which has a sensitivity level of 0 at a spatial frequency of 0 c/mm, a maximum sensitivity level of 1 in the vicinity of a spatial frequency of 0.8 c/mm, a sensitivity level of about 0.4 at a spatial frequency of 2 c/mm, and a sensitivity level of about 0 at a spatial frequency in the range from 6 to 8 c/mm.
  • a model of human visual frequency characteristics is described in detail in "Design of minimum visual modulation halftone patterns" written by J. Sullivan, L. Ray, and R. Miller, IEEE Trans. Syst. Man Cybern., vol. 21, No. 1, 33 - 38 (1991).
  • the low-frequency components extracted by the LPF 40 are IFFTed into low-frequency components in the spatial domain by the IFFT unit 36. Because the produced low-frequency components have intensity variations, an image made up of these low-frequency components and the positions of the threshold candidates th' in the threshold matrix TM are compared with each other, and positions where the low-frequency components are weakest (the values are smallest) are set to threshold candidates th'_hl.
  • positions where the low-frequency components are strongest may be set to threshold candidates th'_sd.
  • step S16 low-frequency components may similarly be extracted, and pixels may be deleted from dots in positions where the low-frequency components are strongest (the values are greatest), of the new threshold candidates th'_hl.
  • pixels may be deleted from dots based on the new thresholds th'_sd in positions where the low-frequency components are weakest (the values are smallest).
  • FIG. 9A shows a dot pattern 120 having a dot percentage of 30% where the dots of a minimum size are 2 ⁇ 2-pixel dots, according to the present embodiment, the dot pattern 120 being generated by the above process.
  • FIG. 9C shows a dot pattern 122 of the conventional 2 ⁇ 2-pixel dot FM screen.
  • FIGS. 9B and 9D show dot patterns 124, 126, respectively, with dark and light areas which are produced by processing the dot patterns 120, 122 with the visual characteristic filter 42 used as the LPF 40.
  • FIGS. 10A and 10B show in perspective respective patterns 128, 130 which represent the dot patterns 124, 126 with dark and light areas.
  • the vertical axis represents dot percentages with white at 0, black at 1.0, and the dot percentage of 30% at 0.30, and the horizontal axes represent pixels. It can be seen that the dot pattern 120 shown in FIG. 9A according to the present embodiment has smaller intensity variations in the dark and light areas and hence smaller amplitudes than the conventional dot pattern 122 shown in FIG. 9C.
  • step S15 or S16 when the dot pattern is IFFTed by the IFFT unit 36 to produce the low-frequency components in the spatial domain, the low-frequency components may further be FFTed by the FFT unit 32, and particular frequency components may be extracted in a descending intensity order by a particular frequency component extractor 44.
  • the extracted particular frequency components may be IFFTed in a descending intensity order to produce images in the spatial domain, and positions where intensity components are weakest, of the positions which do not intensify these images, may be set to threshold candidates th' or threshold candidates th'_hl.
  • a predetermined number of thresholds th may be established on the threshold matrix TM corresponding to positions where dots are newly assigned on the dot pattern.
  • step S17 the dot pattern generated by the determined thresholds th is optimized. This process of optimizing the dot pattern is not required if a high-quality dot pattern has been generated by the processing up to step S16.
  • the process of optimizing the dot pattern may be either one or both of the method disclosed in Japanese Patent No. 3400316 and the process disclosed in Japanese Laid-Open Patent Publication No. 2002-369005.
  • low-frequency components are extracted from the dot pattern generated by the thresholds th_hl.
  • pixels that are placed in positions where the intensity is strongest and pixels that are placed in positions where the intensity is weakest are switched around such that the former pixels will be white pixels and the latter pixels will be blackened pixels, thereby reducing the intensities of the low-frequency components.
  • the blackened pixels have to be pixels attached to the periphery of dots, i.e., pixels held in contact with the periphery of dots, and the threshold th of the blackened pixels is of value equal to the threshold th of the dots.
  • the dot pattern generated by the thresholds th is FFTed, thereafter filtered by the visual characteristic filter 42 and the LPF 40, and then IFFTed into low-frequency components in the spatial domain.
  • the low-frequency components are FFTed to extract frequency components in a descending intensity order.
  • the extracted particular frequency components are IFFTed in a descending intensity order to produce images in the spatial domain, and pixels in positions where intensity components are weakest, of the positions which do not intensify these images and pixels that are placed in positions where the intensity is weakest are extracted and switched around, thereby reducing the intensities of the low-frequency components.
  • the extracted pixels have to be pixels attached to the periphery of dots, and the threshold th of the blackened pixels is of value equal to the threshold th of the dots.
  • a density image corresponding to a dot pattern output from an image output apparatus may be simulated, i.e., predicted, by a density image simulator (predictor) 46, and low-frequency components may be extracted from the density image.
  • a test pattern is actually output from the output system 22, and the density image simulator 46 measures how one dot of the original dot pattern is output on the test pattern with dark and light areas, thereby calculating the dot percentage of a density image close to an actual density image from the dot pattern.
  • An amount of exposure from the shape of the laser beam used in the output system 22 is integrally calculated, and a density image is predicted from the gamma characteristics of the photosensitive material on the printing plate materials EM.
  • a simulation shape for computer calculations of a laser beam for forming 1 ⁇ 1-pixel dots, 2 ⁇ 2-pixel dots, ⁇ on a recording medium such as a film F or the like is determined.
  • the laser beam has a shape close to the Gaussian distribution which can substantially be expressed using a beam diameter that is determined by the maximum value 1/e 2 of the amplitude.
  • the amount of exposure for each dot is calculated from the laser beam and the dot pattern.
  • the amounts of exposure for the respective dots i.e., 1 ⁇ 1-pixel dots, 2 ⁇ 2-pixel dots, ⁇ are converted into densities of the dots using the exposure characteristics, i.e., the gamma characteristics, of the photosensitive material such as a film or the like.
  • a density image density-simulated image
  • Low-frequency components can be extracted from the density image according to the above process using FFT. Actually, low-frequency components that are extracted from a density image can often be more effective to remove noise components, rather than low-frequency components extracted from a dot pattern.
  • thresholds th_sd for the shadow area SD are determined in steps S22 through S28.
  • step S29 the thresholds th_hl determined from the highlight area HL and the thresholds th_sd determined from the shadow area SD are compared with each other for magnitude, and thresholds th_hl and thresholds th_sd are determined until they are of the same value, i.e., until the dot percentage of 50% is achieved.
  • thresholds th_hl and thresholds th_sd are of the same value, the generation of the threshold matrix is finished.
  • FIGS. 11A through 11F show dot patterns 131 through 135, 137, respectively, which are part of dot patterns having dot percentages of 10%, 20%, 30%, 40%, 50%, and 70% that are finally generated by dot pattern generator 18 by comparing the thus generated threshold matrix TM with continuous-tone image data of gray patterns having corresponding dot percentages with the comparator 16.
  • the dot pattern 137 having the dot percentage of 70% may be a pattern that is generated by reversing the white and black areas of the dot pattern 133 having the dot percentage of 30%, or an independently generated pattern.
  • the dot patterns 131 through 135, 137 shown in FIGS. 11A through 11F are generated by selecting the dot-and-dash-line curve nb which represents the accumulated value of the number of new dots in FIG. 4.
  • the dot pattern 131 having the dot percentage of 10% is made up of only 2 ⁇ 2-pixel dots of a minimum size.
  • the dot pattern 132 having the dot percentage of 20% include a reduced proportion of 2 ⁇ 2-pixel dots of a minimum size and pixels (including 4-through 12-pixel dots) corresponding to the dot percentage that are attached to the periphery of the existing dots (2 ⁇ 2-pixel dots). In dot percentages from 25% to 30%, dots of a minimum size are not newly assigned, but pixels are attached to the existing dots, thereby increasing the blackened ratio.
  • dots of a minimum size are determined for a highlight area, and a pattern frequency r in the intermediate tone of a dot pattern is determined (step S1). Based on the pattern frequency r, candidate positions for the dots are determined (step S2). Then, the number Dc of new dots of a minimum size is established at each dot percentage (step S3). Under the limitations of the number Dc of new dots of a minimum size and the pattern frequency r in the intermediate tone, thresholds th for generating optimum dot patterns at respective dot percentages are successively generated (step S4). In this manner, a threshold matrix TM optimum for the output system 22 can be generated.
  • the threshold matrix TM optimum for the output system 22 means a threshold matrix TM which is capable of generating an image where dots are reliably and solidly assigned to a highlight area, and grainness is reduced and a dot gain is small in an intermediate tone area, for example.
  • the threshold array makes it possible to generate dot pattern data Ha where dots of a minimum size which are made up of n pixels (n is at least 1) are placed out of contact with each other when the dot percentage P increases from 0% to a certain dot percentage where the number of dots becomes nearly N ⁇ M/(R/r) 2 . Also, the threshold array makes it possible to generate dot pattern data Ha where pixels are attached to the periphery of the existing dots of a minimum size and the number of dots is not increased after the dot percentage P is more than the certain dot percentage where the number of the dots becomes nearly N ⁇ M/(R/r) 2 .
  • the threshold matrix TM has the dot areas adjusted by attaching pixels to the periphery of the existing dots of a minimum size.
  • One method of generating a threshold matrix according to the present invention is described above.
  • the method can be applied to a method for generating a threshold matrix for generating a general FM screen or a general stochastic screen.
  • a method of assigning a generated threshold matrix is described below.
  • one threshold matrix may be used for M and G printing plates, one threshold matrix for C and R printing plates, and one threshold matrix for Y and B printing plates.
  • one threshold matrix may be used for M and G printing plates, and one threshold matrix for C and orange printing plates.
  • J threshold matrixes having the same size but different threshold arrays may be generated.
  • threshold matrixes for J (J ⁇ 5) color separations requires a heavy workload. Also, it is difficult to handle J color separations in a RIP (Raster Image Processor) system generally using four threshold matrixes for four C, M, Y and K colors. Thus, hereinafter, a simple method of generating and assigning threshold matrixes will be described, in view of Munsell Hue Circle.
  • (printing) plate or “film” will be used in the description.
  • the present invention is not limited to an apparatus, a system, a method, or the like using physical plates or films for reproducing color images.
  • the present invention can be applied to every apparatus, system, method, or the like that reproduces color images by using color separations such as dot pattern data for colors.
  • FIGS. 12A, 12B and 12C show a hue circle HC.
  • the hue circle HC comprises five principle hues (or colors) R (Red), Y (Yellow), G (Green), B (Blue) and P (Purple), which are arranged in a circle (hue circle) so that the circle is equally divided into five. Then, other hues YR (Yellow Red or Orange), GY (Green Yellow), BG (Blue Green or Cyan), PB (Purple Blue) and RP (Red Purple or Magenta) are arranged between those five principle hues.
  • a color component of a hatched and substantially fan-shaped area 302 is defined by boundaries of the color C, the color G color and the color K.
  • the color component of the area 302 can be produced by using a C-plate, a G-plate and a K-plate. It is necessary to assign different threshold matrixes to the C-plate, the G-plate and the K-plate for avoiding shortcomings due to slight displacement between superimposed dots.
  • the threshold matrixes each having a different threshold array for the C-plate, the G-plate and the K-plate are referred to as TMX1, TMX2 and TMX3, and indicated as follows.
  • (C, G, K) (TMX1, TMX2, TMX3)
  • a color component of a hatched and substantially fan-shaped area 304 is defined by boundaries of the color Y, the color G and the color K.
  • the area 304 is on the right side of the area 302.
  • the threshold matrixes TMX2 and TMX3 have already been assigned to the G-plate and the K-plate, respectively.
  • the threshold matrix TMX1 which is different from the threshold matrixes TMX2 and TMX3 assigned to the G-plate and the K-plate but same as the threshold matrix TMX1 assigned to the C-plate, can be assigned to the Y-plate.
  • threshold matrixes for C, M, Y, K, R, G and B can be assigned as indicated below by using the three threshold matrixes TMX1, TMX2 and TMX3 (see FIG. 12C).
  • C, M, Y, K, R, G, B) (TMX1, TMX1, TMX1, TMX3, TMX2, TMX2, TMX2)
  • each of the threshold matrixes converting continuous-tone image data into dot pattern image data, the continuous-tone image data comprising at least data for J colors (J ⁇ 5) including C, M, Y and K, the dot pattern image data comprising data for J plates in which screen ruling or screen angle is not defined, a threshold matrix TMX3 with a first threshold array is assigned to a K-plate (a threshold matrix for a K-plate); a threshold matrix TMX1 with a second threshold array is assigned to, e.g., a C-plate that is one of the J plates other than the K-plate; and a threshold matrix TMX2 with a third threshold array is assigned to a G-plate and a B-plate, G and B are adjacent to C in a hue circle, the number of the threshold matrixes assigned to the J plates is made as small as possible.
  • the three different threshold matrixes TMX3, TMX1 and TMX2 are assigned to a plate for K-color, a plate for another color other than K, and a plate for a color adjacent to the other color in the hue circle HC, respectively.
  • the hue circle HC generally, a color component of a substantially fan-shaped area defined by boundaries of K-color, a color other than K, and another color adjacent to the other color can be reproduced by mixing these three colors.
  • threshold matrixes In assigning threshold matrixes, if the number of the threshold matrixes is made as small as possible on condition that the threshold matrixes for adjacent colors are not the same as each other, workload of generating the threshold matrixes can be reduced.
  • the J plates are made up of a C-plate, an M-plate, a Y-plate, a K-plate, an R-plate, a G-plate and a B-plate.
  • C is adjacent to G
  • G is adjacent to Y
  • Y is adjacent to R
  • R is adjacent to M
  • M is adjacent to B
  • B is adjacent to C, other than K, as shown in FIG. 12C (i.e., colors (O), (GY), (PB) and (P) in FIG. 12C are not considered in this example).
  • the threshold matrix TMX1 is assigned to one of the C-plate, Y-plate and the M-plate.
  • C is not adjacent to Y
  • Y is not adjacent to M
  • M is not adjacent to C.
  • the threshold matrix TMX2 is assigned to the G-plate, the R-plate and the B-plate.
  • G is not adjacent to R
  • R is not adjacent to B
  • B is not adjacent to G. Accordingly, for the plates for the colors of C, M, Y, K, R, G and B, only three threshold matrixes TMX1, TMX2, TMX3 each having a different threshold array are sufficient. Since the total number of the threshold matrixes is smaller than the number of the plates, workload of generating the threshold matrixes can be reduced.
  • each of the threshold matrixes TMXc, TMXm, TMXy and TMXk each having a different threshold array for plates for colors C, M, Y and K converts continuous-tone image data input with a tone value m comprising at least data for J colors (J ⁇ 5) into v-valued dot pattern image data comprising data for J plates in which screen ruling or screen angle is not defined.
  • One color other than C, M, Y or K is chosen, and one of the threshold matrixes TMXc, TMXm and TMXy for the C-plate, the M-plate and the Y-plate is assigned to a plate for the color.
  • the threshold matrix for one of C, M and Y that is not adjacent to the color in the hue circle is chosen. That is, the threshold matrix assigned to the plate for the color other than C, M, Y or K is different from the threshold matrix for C, M or Y that is adjacent to the color in the hue circle. Accordingly, the number of the threshold matrixes for multicolor plates for reproducing a color image with five or more plates may generally be four, i.e., the threshold matrixes TMXc, TMXm, TMXy and TMXk for the C-plate, the M-plate, the Y-plate and the K-plate. Thus, the threshold matrixes that do not cause shortcomings in superimposing images can be generated and assigned with a light workload.
  • Second Assigning Method (Assignment of threshold matrixes having the same threshold array to plates for hues that are complementary to each other in a hue circle):
  • a 7-color printing process using plates for C, M, Y, K, R, G and B is considered.
  • the hue circle HC shown in FIG. 13 for smooth reproduction of a color gradation of hues in the vicinity of G, in practice, it is preferable to mix C and Y.
  • it is much preferable to assign different threshold matrixes to the C-plate and the Y-plate though the same threshold matrix TMX1 is assigned to the C-plate and Y-plate in the first assigning method.
  • threshold matrixes TMXc, TMXm, TMXy and TMXk having different threshold arrays are prepared.
  • these threshold matrixes TMXc, TMXm, TMXy and TMXk can be assigned to for the C-plate, the M-plate, the Y-plate and the K-plate, respectively.
  • C, M, Y, K (TMXc, TMXm, TMXy, TMXk)
  • the threshold matrix TMXm for the M-plate can be used for the G-plate.
  • the threshold matrixes for the plates of C, M, Y and K colors can be assigned to each of the plates of C, M, Y, K, R, G and B colors as follows.
  • (C, M, Y, K, R, G, B) (TMXc, TMXm, TMXy, TMXk, TMXc, TMXm, TMXy)
  • This assignment means that: the threshold matrix TMXc for the C-plate is assigned to the color R complementary to the color C; the threshold matrix TMXm for the M-plate is assigned to the color G complementary to the color M; and the threshold matrix TMXy for the Y-plate is assigned to the color B complementary to the color Y.
  • a 6-color printing process using plates for C, M, Y, K, O and G is considered.
  • the threshold matrixes TMXc, TMXm, TMXy and TMXk having different threshold arrays are assigned to for the C-plate, the M-plate, the Y-plate and the K-plate, respectively.
  • the threshold matrix TMXc for the C-plate is assigned to the color O substantially complementary to the color C
  • the threshold matrix TMXm for the M-plate is assigned to the color G complementary to the color M.
  • the color O is considered to be substantially complementary to the color C since it is far from the color C in the hue circle HC.
  • the threshold matrixes for the plates of C, M, Y and K colors can be assigned to each of the plates of C, M, Y, K, O and G colors as follows.
  • (C, M, Y, K, O, G) (TMXc, TMXm, TMXy, TMXk, TMXc, TMXm)
  • each of the threshold matrixes TMXc, TMXm, TMXy and TMXk converting continuous-tone image data into dot pattern image data, the continuous-tone image data comprising at least data for J colors (J ⁇ 5) including C, M, Y and K, the dot pattern image data comprising data for J plates in which screen ruling or screen angle is not defined, a first threshold matrix TMXk with a first threshold array is assigned to a K-plate (a threshold array for a K-plate); and a second threshold matrix with a second threshold array is assigned to plates for colors other than K, the colors other than K are complementary to each other.
  • threshold matrixes having the same threshold array are used for the hues that are complementary to each other, unstable color reproduction or unevenness or irregularity of hue or shade in the image due to less superimposition of dots by screen displacement does not occur. This is because the colors that are complementary to each other are seldom mixed. If the colors that are complementary to each other are mixed, the mixed color is merely gray. Further, workload of generating the threshold matrixes can be reduced since the total number of the threshold matrixes is smaller than the number of printing plates for respective colors.
  • the J plates are made up of a C-plate, an M-plate, a Y-plate, a K-plate, an R-plate, a G-plate and a B-plate.
  • a threshold matrix TMXc for the C-plate is assigned to the C-plate
  • a threshold matrix TMXm for the M-plate is assigned to the M-plate
  • a threshold matrix TMXy for the Y-plate is assigned to the Y-plate
  • a threshold matrix TMXk for the K-plate is assigned to the K-plate.
  • These four threshold matrixes TMXc, TMXm, TMXy and TMXk are different from each other.
  • C is adjacent to G
  • G is adjacent to Y
  • Y is adjacent to R
  • R is adjacent to M
  • M is adjacent to B
  • B is adjacent to C.
  • the threshold matrix TMXm for the M-plate is assigned to the G-plate
  • the threshold matrix TMXc for the C-plate is assigned to the R-plate
  • the threshold matrix TMXy for the Y-plate is assigned to the B-plate, since G is complementary to M, R is complementary to C, and B is complementary to Y.
  • the threshold matrixes TMXc, TMXm, TMXy and TMXk each having a different threshold array for colors C, M, Y and K can be used.
  • the workload of generating the threshold matrixes can be reduced since the total number of the threshold matrixes is smaller than the number of printing plates for respective colors. In this way, it is possible to handle threshold matrixes for printing plates for C, M, Y, K, R, G and B colors in a RIP system etc. generally using four threshold matrixes for four C, M, Y, K colors.
  • the J plates are made up of a C-plate, an M-plate, a Y-plate, a K-plate, an O-plate and a G-plate. Then, a threshold matrix TMXc for the C-plate is assigned to the C-plate, a threshold matrix TMXm for the M-plate is assigned to the M-plate, a threshold matrix TMXy for the Y-plate is assigned to the Y-plate, and a threshold matrix TMXk for the K-plate is assigned to the K-plate. These four threshold matrixes TMXc, TMXm, TMXy and TMXk are different from each other.
  • C is adjacent to G
  • G is adjacent to Y
  • Y is adjacent to O
  • O is adjacent to M
  • M is adjacent to C.
  • the threshold matrix TMXm for the M-plate is assigned to the G-plate and the threshold matrix TMXc for the C-plate is assigned to the O-plate, since G is complementary to M, and O is complementary to C.
  • the threshold matrixes TMXc, TMXm, TMXy and TMXk each having a different threshold array for colors C, M, Y and K can be used.
  • the workload of generating the threshold matrixes can be reduced since the total number of the threshold matrixes is smaller than the number of printing plates for respective colors. In this way, it is possible to handle threshold matrixes for printing plates for C, M, Y, K, O and G colors in a RIP system etc. generally using four threshold matrixes for four C, M, Y, K colors.
  • one of the threshold matrixes is generated by changing a reading method of thresholds placed in the threshold array in other of the threshold matrixes.
  • a threshold array in the threshold matrix can be changed, and time for generating a threshold matrix can be significantly reduced, compared with a threshold matrix generated from nothing.
  • Japanese Laid-Open Patent Publication No. 2004-64473 discloses that a reading method of a threshold matrix is changed for generating another threshold matrix.
  • FIG. 15 A threshold matrix TMorg for a certain color plate used for a FM screen is generated, and the matrix size thereof is 10 ⁇ 10. Then, as shown in FIG. 16, four threshold matrixes TMorg are arranged.
  • a first reading address i.e., a position for starting reading thresholds
  • Rs (x, y) in view of x-axis (row) and y-axis (column) is changed from Rs (1, 1) to Rs (5, 4)
  • a new threshold matrix TMother having another threshold array as shown in FIG. 17 is obtained easily.
  • reading method for changing reading method, as well as changing the first reading address as above, other reading methods may be utilized, such as changing the reading direction of the threshold array for rearrangement (e.g., reading the threshold array from the right to the left) or rotating the threshold array for rearrangement by, e.g., 90 degrees.
  • the sizes of threshold matrixes for C-plates, M-plates, Y-plates and K-plates are changed, so that unnecessary periodic pattern in the dot pattern can be reduced.
  • the threshold matrixes TMX are arranged for use as a superthreshold matrix STMXa corresponding to N ⁇ N pixels that are output from the output system 22 and that are formed on the printing plate PP or the like. Even when the threshold matrixes each having a different threshold array are used, some repetitive patterns at a matrix size of the threshold matrix TMX for each plate are emphasized by superimposing four plates for C, M, Y and K, if the sizes of the threshold arrays are the same.
  • threshold matrixes TX1 through TX4 each having a different matrix size for the plates for C, M, Y and K are prepared.
  • the relationships between the threshold matrixes TX1 through TX4 and the plates for C, M, Y and K may be determined desirably.
  • the threshold matrixes TX1 through TX4 have matrix sizes: N1 ⁇ N1 (shown in a solid line), N2 ⁇ N2 (shown in a dashed line), N3 ⁇ N3 (shown in a dot-and-dash line) and N4 ⁇ N4 (shown in a two-dot-and-dash line), respectively (N2 ⁇ N1 ⁇ N4 ⁇ N3).
  • the matrix size of N1 ⁇ N1 means that the array has the N1 ⁇ N1 thresholds corresponding to the N1 ⁇ N1 pixels.
  • the threshold matrixes TMX1 through TX4 are arranged for use as a superthreshold matrix STMXb for the plates for C, M, Y and K.
  • STMXb superthreshold matrix
  • any periodic pattern is not emphasized in superimposing the four plates for C, M, Y and K since matrix sizes of N ⁇ N for the four plates are different from each other.
  • the matrix sizes of N ⁇ N for the plates for C, M, Y and K are all different from each other; however, some of the matrix sizes of N ⁇ N may be the same.
  • N1, N2, N3, N4 (128, 144, 160, 176), (128, 160, 192, 224), (256, 320, 384, 448), (512, 576, 640, 704), and so on.
  • Threshold matrixes thus generated may be used as follows:
  • RGB image data captured by a digital camera 202 as an image capturing unit or RGB image data (or CMYK image data) read by a plate input machine 204 as a scanner (image reader) are supplied to an RIP (Raster Image Processor) 206, which converts the RGB image data into CMYK image data.
  • RIP Raster Image Processor
  • the RIP 206 stores in its storage unit such as a hard disk or the like data of threshold matrixes TM (threshold matrix data) generated by the threshold matrix generating apparatus 20 and supplied through an optical disk 208 serving as a storage unit such as a CD-R or the like or through a communication link.
  • TM threshold matrix data
  • the CMYK dot pattern data are then sent to a DDCP (Direct Digital Color Proofer) 210, which produces a print proof PRa on a sheet of paper.
  • the DDCP 210 allows the operator to confirm noise components and printing quality on the print proof PRa before the image data are processed by a printing press 220.
  • the sheet of paper used by the DDCP 210 may be a sheet of printing paper used by the printing press 220.
  • the RIP 206 delivers the CMYK dot pattern data to a color ink jet printer 20c1 which produces a printing proof PRb on a sheet of paper or a color electrophotographic printer 20c2 which produces a printing proof PRc on a sheet of paper.
  • the CMYK dot pattern data are also sent to the exposure unit 26 which serves as a filmsetter or a platesetter in the output system 22 such as a CTC apparatus or the like. If the exposure unit 26 is a filmsetter, the automatic developing machine 28 generates a film F. The film F is superposed on a printing plate material, and exposed to light by a planar exposure unit (not shown), producing a printing plate PP. If the exposure unit 26 is a platesetter as shown in FIG. 1, then the automatic developing machine 28 directly outputs a printing plate PP.
  • the exposure unit 26 is supplied with printing plate materials EM from a magazine 212 of photosensitive materials (including plate materials).
  • CMYK printing plates PP are mounted on plate cylinders (not shown) in a K-plate printer 214K, a C-plate printer 214C, an M-plate printer 214M, and a Y-plate printer 214Y of the printing press 220.
  • the CMYK printing plates PP are pressed against a sheet of printing paper supplied from a printing paper supply unit 216 to transfer the inks to the sheet of printing paper, thereby producing a printed material PM on which a color image is reproduced.
  • the RIP 206 supplies the CMYK dot pattern data directly through a communication link, and the printing plates mounted on the plate cylinders are exposed to record image data and then developed directly into printing plates PP.
EP05004587A 2004-03-05 2005-03-02 Schwellenmatrix zur Erzeugung von Rastern, Verfahren zur Erzeugung und zur Zuweisung solcher Matrix Withdrawn EP1571826A3 (de)

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US7492483B2 (en) 2009-02-17

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